(1) Field of the Invention
The present invention relates to a method for measuring, effectively and with high accuracy, characteristics of a wavelength division multiplexing optical amplifier to be utilized for optical communications, and a characteristic measuring system applied with the method.
(2) Description of the Related Art
In a WDM optical amplifier for amplifying a wavelength division multiplexed (WDM) signal light containing a plurality of signal lights of different wavelengths, in order to measure and evaluate characteristics thereof, it is necessary to measure accurately a signal light level and a noise light level of each signal light wavelength. To be specific, for example, to obtain gain, noise figure and the like, being typical indexes for indicating characteristics of the optical amplifier, an input signal light level, an output signal light level and an optical noise (spontaneous emission light) level corresponding to each signal light wavelength must each be obtained. It is relatively easy to measure the input signal light level and output signal light level of each wavelength. However, regarding measurement of spontaneous emission light level, it is difficult to measure a spontaneous emission light level corresponding to each wavelength independently, since light output from the optical amplifier contains both an amplified signal light and spontaneous emission light.
For prior art to measure a spontaneous emission light level of an optical amplifier, for example, measuring methods such as a pulse method, a probe method, an interpolation method and the like are known. The pulse method is, for example as described in Japanese Unexamined Patent Publication No. 8-248454, for measuring an output light level at a time of no signal light input, by modulating a signal light at a cycle sufficiently shorter than an atomic lifetime or carrier lifetime of an optical amplification medium. Furthermore, the probe method is a method wherein signal lights of the smaller number of wavelengths than all of the signal light wavelengths in a measurement wavelength band is used to create a simulated saturation state in the optical amplifier of when signal lights of all wavelengths are input, to make the spectrum thereof a spontaneous emission light spectrum. Moreover, the interpolation method is a method for estimating a spontaneous emission light spectrum from an output light spectrum of an optical amplifier by interpolation.
However, the conventional characteristic measuring methods as mentioned in the above have the following problems in terms of accuracy of measured values, measurement speed, simplicity of measuring system and the like. That is to say, a problem of the pulse method is that the measuring system is complicated. For example, as shown in FIG. 15, the pulse method requires a complicated measuring system where the optical paths when measuring the input light power, output light power and spontaneous emission light power are all different. Furthermore, since it is required to compensate for differences in losses in the optical paths, there is also a drawback in that the measurement operation is troublesome. Moreover, since a loss of the modulator is great, the power of light that reaches the measuring device becomes small, so that there is also a problem in that a ratio (optical SN ratio) of signal light to noise light is deteriorated.
Furthermore, a problem of the probe method is that the measurement time becomes long. For example, as shown in FIG. 16, in the probe method, a series of operations is repeated in which, while maintaining a reverse distribution state of the optical amplifier, the level of signal light is measured after the wavelength of a weak probe light is adjusted to a point of wavelength grid to be measured. Hence, the measurement time gets longer in proportion to the number of wavelengths in a WDM signal light.
Moreover, a problem of the interpolation method is that measured value lacks repeatability and accuracy. For example, as shown in FIG. 17, in the interpolation method, the spontaneous emission light level at a wavelength to be measured is not obtained by actual measurement but by estimation. Therefore, there is a possibility that dispersion occurs in the estimated values caused by persons performing measurement or by selection methods for an approximation curve used for interpolation, and repeatability is worsened. Moreover, the spontaneous emission light level to be estimated also depends, for example as shown in FIG. 18, on the performance (especially resolution and dynamic range) of an optical spectrum analyzer. Therefore, if the measurement accuracy is to be increased, the requirement with respect to performance of the measuring device becomes more critical.
The present invention addresses the abovementioned problems, with an object of providing a characteristic measuring method and a characteristic measuring system of a WDM optical amplifier, capable of performing high speed and accurate measurement of characteristics of an optical amplifier by a measuring system with a simple construction.
To achieve the abovementioned object, with one aspect of a characteristic measuring method of a WDM optical amplifier according to the present invention, in a characteristic measuring method of an optical amplifier for amplifying a WDM signal light in which a plurality of signal lights of different wavelengths are multiplexed, firstly a WDM signal light that does not include a signal light of a first wavelength, being one of the plurality of signal lights, and has the total power approximately equal to the total power of the WDM signal light is input to the optical amplifier. Then, the power of the signal light corresponding to the first wavelength among output signal lights of the optical amplifier is measured, to thereby detect the power of spontaneous emission light corresponding to the first wavelength, from the optical amplifier of when the WDM signal light is input.
According to such a characteristic measuring method, characteristic measurement of an optical amplifier is performed using a WDM signal light from which the signal light of the first wavelength is removed and total power adjusted, thereby enabling actual measurement of the power of spontaneous emission light corresponding to the first wavelength contained in the light output from the optical amplifier.
In one method based on the abovementioned characteristic measuring method, firstly a plurality of signal lights corresponding to respective signal light wavelengths in a measurement wavelength band are divided into at least two or more groups such that signal lights of adjacent wavelengths are in different groups, and the power of each signal light is adjusted such that the total power of the signal lights in each group is approximately equal to a preset reference value. Then, a WDM signal light containing the multiplexed signal lights is input in turn for each group to the optical amplifier, to measure a spectrum of the output light from the optical amplifier for each group. Then, based on these spectrum measurement results, the output signal light power and the spontaneous emission light power of each signal light wavelength in a measurement wavelength band are judged.
According to such a characteristic measuring method, a WDM signal light amplified by the optical amplifier is divided into a plurality of groups for measurement, so that for all of the signal light wavelengths in the measurement wavelength band, it is possible to actually measure the respective powers of the output signal light and the spontaneous emission light that are contained in the light output from the optical amplifier.
Furthermore, for the abovementioned characteristic measuring method, a spectrum of the WDM signal light input to the optical amplifier may be measured, to judge the input signal light power of each signal light wavelength in the measurement wavelength band. By using the input signal light power, the output signal light power and the spontaneous emission light power judged for each signal light wavelength in the measurement wavelength band, it is possible, for example, to calculate gain, noise figure and the like of the optical amplifier.
Moreover, for the abovementioned characteristic measuring method, it is preferable that the distribution of signal light power in the measurement wavelength band in each group is approximately uniform with respect to the wavelength direction. In this manner, the generating condition of spontaneous emission light in the optical amplifier at the time of measurement for each group becomes constant, which enables more accurate measurement of spontaneous emission light power.
According to another aspect of the characteristic measuring method of a WDM optical amplifier according to the present invention, in a characteristic measuring method of an optical amplifier for amplifying a WDM signal light in which a plurality of signal lights of different wavelengths are multiplexed, firstly rearward spontaneous emission light emitted from a signal light input terminal is ejected from spontaneous emission light generated in the optical amplifier input with a WDM signal light, and a spectrum of the rearward spontaneous emission light ejected, and a spectrum of output light containing output signal light and forward spontaneous emission light emitted from a signal light output terminal of the optical amplifier are respectively measured. Then, the spectrum of the rearward spontaneous emission light is fitted to the measured spectrum of the output light from the optical amplifier, and based on the fitted spectrum of the rearward spontaneous emission light, the forward spontaneous emission light power of each signal light wavelength in a measurement wavelength band is judged, and also based on the spectrum of the output light from the optical amplifier, the output signal light power of each signal light wavelength in the measurement wavelength band is judged.
According to such a characteristic measuring method, the fitting process is performed on the spectrum of the actually measured rearward spontaneous emission light, so that a spectrum of the forward spontaneous emission light contained in the output light from the optical amplifier can be estimated with high accuracy. Therefore, it is possible to judge accurately the forward spontaneous emission light power of each signal light wavelength.
For the abovementioned characteristic measuring method, to be specific, the fitting of the rearward spontaneous emission light to the spectrum of the output light from the optical amplifier may be performed using spectrum data of a wavelength region excluding respective signal light wavelengths inside the measurement wavelength band. Alternatively, it may be performed using spectrum data of a wavelength region outside of the measurement wavelength band.
Furthermore, for the abovementioned characteristic measuring method, a relationship between the dependency of the rearward spontaneous emission light on the wavelength and the dependency of the forward spontaneous emission light on the wavelength may be obtained in advance, to correct based on the obtained relationship, the measurement result of the spectrum of the rearward spontaneous emission light. In this manner, differences in wavelength characteristics of the rearward spontaneous emission light and wavelength characteristics of the forward spontaneous emission light are compensated for, so that it is possible to increase measurement accuracy.
A characteristic measuring system of a WDM optical amplifier according to the present invention is applied with each aspect of the characteristic measuring method of the present invention as mentioned above. In such a characteristic measuring system, it is possible to measure the characteristics of optical amplifiers accurately using a measuring system with a simple construction and low loss.
Other objects, features and advantages of this invention will become apparent in the following description of embodiments in relation to the attached drawings.